Following around six months of construction, a small hydroelectric power plant on the River Köberlbach, which in principle is completely new, is set to start operating again in the community of Gaishorn am See in Upper Styria.

As part of the transformation, the plant, which was built back in the 1920s, is being almost completely reconstructed. Apart from a section of the penstock that was replaced around 15 years ago and the power plant building, all plant components are being renewed from scratch. Thanks to an increase in the drop and extraction water quantity, the plant, which is now equipped with a 3-nozzle Pelton turbine, can generate significantly more electricity. The pipeline, which in some sections runs with a drop of 100%, was fully laid over a length of more than 1,000 m in highly resistant ductile cast-iron pipes with a restrained and tension-proof design. The pipe material, which was provided by the professional sales company Geotrade from Upper Austria, proved to be a very successful choice during the winter construction work with ground conditions and weather that both presented their challenges.

The first commissioning of Köberlbach power plant, which is located in the northern part of the market town of Gaishorn am See in the district of Liezen, took place more than 90 years ago. According to records, electricity was produced with the discharge plant for the first time back in 1926. Following the turn of the millennium, its ownership transferred from the community of Gaishorn to a new tenure. As part of this takeover, the old turbine was replaced with a modern machine set back in 2003. Around ten years later, during the official procedure to renew the water rights, an increase in the extraction water quantity of 50 l/s to a total of 170 l/s was granted. The increased extraction water quantity and the increase in the drop that was also approved finally made new construction economically viable. Apart from the power plant building and an around 160 m section of the penstock that was replaced back in 2003, all plant components are being renewed from scratch.

CHALLENGING CONSTRUCTION WORK
After receiving the final approval and completing the tendering procedure, the construction work began with the laying of the penstock in September of last year. This was followed shortly after by the excavation and concrete work on the water catchment, which is designed as a Tyrolean weir. This was completely reconstructed further upstream to increase the drop. All of the structural and civil engineering work was carried out by Gottfried Guster GmbH from Upper Styria, which has proven itself many times in the hydropower sector. “Despite the less than ideal weather conditions with snow and freezing cold, it proved possible to lay the whole of the penstock stretching for a total of 1,026 m in a construction time of around three months. An additional challenge was the geological conditions, which were difficult in some places, and sections of terrain with a gradient of up to 100%,” explains master builder Gottfried Guster. Further, the route chosen for the pipeline required the creation of a river underpass and two underground crossings of a forest track. As a result of the challenging soil conditions – the area of the project is located in geological terms in what is known as the greywacke zone of the Alps – the whole pipeline was laid with a DN400 diameter with a restrained and tension-proof design. Although the construction work was carried out in conditions that were not ideal, the pipeline, which drops down 312 m in total, was fully laid by the end of November.

RESTRAINED AND TENSION-PROOF DESIGN
The minimal yet constant movement of the ground does not present an issue thanks to the resistant pressure pipe system made from ductile cast iron which is certified according to ÖNORM 545. A socket joint which is restrained against longitudinal forces and is secured by means of a bolt is used as a high-strength pipe connection. All of the pipe material was provided by the Upper Austrian expert in pipe distribution from the Mühlviertel region, Geotrade Handelsges.m.b.H. As well as their robust material properties, according to Geotrade managing director Franz Leitner the pipes from the manufacturer SVOBODNY SOKOL also offer a persuasive choice with a whole host of other benefits. For example, the cast-iron pipe is clad or covered with high-quality coatings on both the inside and outside. On the outside the pipe is given a zinc or zinc/aluminium oxide coating and is thus also suitable for laying in aggressive soils. Additional protection is delivered by another outer layer based on bitumen. By contrast, the inner surface of the pipe is completely smooth and consists of a cement mortar lining. As well as the excellent flow conditions, the cement mortar provides both an active and a passive protective effect. The cement mortar coating even enables a “self-healing process” for hairline cracks or microscopic damage. Defective spots, which may occur during installation or transport, for example, are sealed up automatically as a result of a chemical reaction which occurs when the works water comes into contact with the permeable cement jacket and the cast iron.

COMMISSIONING IS IMMINENT
In February, large amounts of snow and very cold temperatures well below freezing meant that a stop to construction work for around two weeks was unavoidable. After this enforced stoppage and the finishing of some concrete work on the weir that is still outstanding, the plant will start operating soon. Following this complete transformation, a horizontal 3-nozzle Pelton turbine from ANDRITZ Hydro will now almost treble the power output compared to the previous installation with a speed of 1,000 rpm. When the snow starts to thaw, which usually happens in April, the turbine coupled to a synchronous generator from Hitzinger will be able to deliver a bottleneck output of 430 kW. All of the electricity which is produced will be fed into the public grid. This new construction also more than doubles the average annual production from a previous level of around 700,000 kWh to 1,600,000 kWh in the future.